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Abstract:

A solar power system may include at least one reflector The reflector may
have a surface configured to convert a first part of the sunlight
incident thereon to electrical power The surface of the reflector may
also be configured to reflect a second part of the sunlight incident
thereon The at least one reflector may be configured to direct the second
part of the sunlight incident thereon to a solar receiver A power
management system may also be provided The power management system may
receiver electrical power derived from the first part from the reflector.

Claims:

1. An apparatus for use in a solar concentrating system, comprising:a
receiving member having layers configured respectively for reflecting and
converting solar energy received thereby, the layers being in a mutually
overlying arrangement and configured such that a first part of the solar
received thereby is converted to energy of another form and a second part
is reflected to a solar energy receiver; andat least one energy power
output configured to convey converted solar energy from the receiving
member.

2. The apparatus of claim 1, further comprising a drive system configured
to aim the member, the drive system being configured to receive energy
from the energy output.

3. The apparatus of claim 1, wherein the layers include a photovoltaic
conversion layer and the energy of another form is electrical energy.

4. The apparatus of claim 1, further comprising a drive system configured
to aim the receiving member such that solar energy is received normal to
the layers at a first time and to selectively aim the receiving member
such that the second part is reflected to a solar energy receiver at a
second time.

5. The apparatus of claim 1, wherein the layers include a photovoltaic
converting layer and a reflecting layer.

6. The apparatus of claim 1, wherein the layers include a waveband filter
layer, a photovoltaic converting layer, and a reflecting layer.

7. The apparatus of claim 1, wherein the layers include a photovoltaic
converting layer and a reflecting layer with variable reflectivity.

8. The apparatus of claim 1, wherein the layers include a photovoltaic
layer and at least one of a reflecting layer and a dichroic filter layer.

9. The apparatus of claim 1, wherein the layers include a filter layer and
a photovoltaic layer having a range of wavelengths over which the
photovoltaic layer exhibits peak conversion efficiency, the filter layer
being configured to reflect light of wavelengths outside said range of
wavelengths and to transmit light of wavelengths within said range of
wavelengths.

10. A method for controlling at least one heliostat in a solar power
system, the at least one heliostat having a reflecting layer and a
conversion layer arranged such that such that a given light ray passes
through both layers, the reflecting layer reflecting a portion of
incident solar radiation, the conversion layer generating electrical
power from another portion of the incident solar radiation, the method
comprising:at a first time, aiming the at least one heliostat so as to
reflect incident solar radiation onto to a solar receiver; andat a second
time, aiming the at least one heliostat toward a different direction
responsively to at least one of an excess flux condition, a low direct
insolation condition, a non-steady state operating condition of a boiler,
and a condition of non-uniform illumination of the conversion layer.

11. The method of claim 10, wherein the second time includes a time when
insolation levels are below a predetermined threshold for use by the
solar receiver.

12. The method of claim 10, wherein the second time includes a time when
the at least one heliostat contributes more to electricity production by
aiming the at least one heliostat directly at the sun than by reflecting
incident solar radiation to the solar receiver.

13. A solar power system, comprising:at least one reflector having a
surface configured to convert a first part of the sunlight incident on
the surface to electrical power and to reflect a second part of the
sunlight incident on the same surface;the at least one reflector being
configured to direct the second part of the sunlight incident thereon to
a solar receiver;a power management system configured to receive
electrical power derived from the first part from the reflector.

14. The system of claim 13, wherein the solar receiver includes a
photovoltaic converter.

15. The system of claim 13, further comprising multiple heliostats,
wherein the reflector receives sunlight from the multiple heliostats.

16. The system of claim 13, wherein the reflector is configured such that
the first part has a first wavelength distribution and the second part
has a second wavelength distribution.

17. The system of claim 13, wherein the reflector has alternating
reflecting and photovoltaic conversion layers that are configured to
permit the partial transmission of light through at least one of the
layers to another of the layers.

18. The system of claim 13, further comprising a controller configured to
selectively aim the reflector such that it is normal to incident sunlight
at times when an angle formed by the receiver, the reflector and the sun,
with the reflector at the angle vertex, is greater than a predefined
angle.

19. The system of claim 13, further comprising a controller configured to
selectively aim the reflector such that it is normal to incident sunlight
at times at times when the power production of the system peaks due to
peak insolation levels and to reflect sunlight to the solar receiver at
other times.

20. The system of claim 13, further comprising a controller configured to
selectively aim the reflector such that it is normal to incident sunlight
at times when the total amount of flux available to the receiver is above
a certain level.

21. The system of claim 13, wherein the reflector surface includes a
reflecting surface and a photovoltaic layer on a transparent substrate
overlying the reflecting surface.

22. The system of claim 13, further including a controller configured to
selectively aim the reflector such that it reflects sunlight onto the
receiver or faces the reflector surface toward the sun responsively to at
least one of parameters indicating instant electricity production of the
system, revenue or profit generation of the system, instant energy
conversion efficiency of the system, alternatively electricity production
of the system, and revenue generation or profit generation of the system
cumulated over a time period

23. The system of claim 13, further comprising non-photovoltaic
heliostats, the reflector being one of multiple photovoltaic heliostats,
the non-photovoltaic heliostats being positioned relative to the receiver
so as to form less acute angles with the sun and the receiver during a
greater fraction of a production period of the year than the photovoltaic
heliostats, whereby the positions of the photovoltaic heliostats and
non-photovoltaic heliostats are chosen such that the angle of incidence
on the photovoltaic heliostats tends to be lower while reflecting
sunlight to the receiver than the non-photovoltaic heliostats.

24. The system of claim 13, further comprising non-photovoltaic
heliostats, the reflector being one of multiple photovoltaic heliostats,
each having an optical acceptance angle, the non-photovoltaic heliostats
being located in positions such that, when reflecting sunlight toward the
receiver, they form angles with the sun and the receiver during a
substantial fraction of a production period that is lower than the
optical acceptance angle, whereby the positions of the photovoltaic
heliostats and non-photovoltaic heliostats are chosen to provide a
highest possible utilization factor of the photovoltaic conversion
capability of the photovoltaic heliostats.

[0002]This invention relates generally to the conversion of solar
radiation to electric power, and more particularly to a solar power
system with reflecting and conversion means.

SUMMARY OF THE INVENTION

[0003]The invention provides an efficient system for the conversion of
solar radiation to thermal and electric energy, as well as components,
articles of manufacture, and other technological improvements, and
methods for using them.

[0004]In an embodiment of the present invention, an apparatus for use in a
solar concentrating system may include (i.e., comprise) a receiving
member having (i.e., comprising) layers configured respectively for
reflecting and converting solar energy received thereby. The layers may
be in a mutually overlying arrangement and configured such that a first
part of the solar received thereby is converted to energy of another form
and a second part is reflected to a solar energy receiver. The apparatus
may also include at least one energy power output configured to convey
converted solar energy from the receiving member.

[0005]In another aspect, the apparatus may also include a drive system
configured to aim the receiving member. The drive system may be
configured to receive energy from the energy power output.

[0006]In another aspect, the layers of the apparatus may include a
photovoltaic conversion layer. The energy of another form may be
electrical energy.

[0007]In another aspect, the apparatus may also include a drive system
configured to aim the receiving member such that solar energy is received
normal to the layers at a first time and to selectively aim the receiving
member such that the second part is reflected to a solar energy receiver
at second time.

[0008]In another aspect, the layers may include a photovoltaic converting
layer and a reflecting layer. In yet another aspect, the layers may
include a photovoltaic that is at least partly transparent and a
reflecting layer. In yet another aspect, the layers may include a
waveband filter layer, a photovoltaic converting layer, and a reflecting
layer. In still another aspect, the layers may include a dichroic filter
layer, a photovoltaic converting layer, and a reflecting layer. In
another aspect, the layers may include a photovoltaic converting layer
and a reflecting layer with variable reflectivity. In another aspect, the
layers may include a photovoltaic layer and at least one of a reflecting
layer and a dichroic filter layer. In still another aspect, the layers
may include a filter layer and a photovoltaic layer having a range of
wavelengths over which the photovoltaic layer exhibits peak conversion
efficiency, the filter layer being configured to reflect light of
wavelengths outside said range of wavelengths and to transmit light of
wavelengths within said range of wavelengths.

[0009]In another aspect, the receiving member may have a dish-shaped
configuration to concentrate the solar energy onto a solar energy
receiver.

[0010]Another embodiment of the present invention may include a method for
controlling at least one heliostat in a solar power system. The at least
one heliostat may have a reflecting layer and a conversion layer arranged
such that light is incident on both layers simultaneously. Alternately,
the at least one heliostat may have a reflecting layer and a conversion
layer arranged such that a light ray is capable of passing through both
layers. The reflecting layer may reflect a portion of incident solar
radiation. The conversion layer may generate electrical power from
another portion of the incident solar radiation. The method may include
aiming the at least one heliostat so as to reflect incident solar
radiation onto to a solar receiver at a first time. The method may
further include aiming the at least one heliostat toward a different
direction responsively to at least one of an excess flux condition, a low
direct insolation condition, a non-steady state operating condition of a
boiler, and a condition of non-uniform illumination of the conversion
layer at a second time.

[0011]In another aspect, the aiming the at least one first heliostat
toward a different direction may result in a greater conversion
efficiency for the conversion layer.

[0012]In another aspect, the second time may include a time when
insolation levels are below a predetermined threshold for use by the
solar receiver.

[0013]In yet another aspect, the second time may include a time when the
at least one heliostat contributes more to electricity production by
aiming the at least one heliostat directly at the sun than by reflecting
incident solar radiation to the solar receiver.

[0014]Another embodiment of the present invention may include a solar
power system. The solar power system may include at least one reflector
having a surface configured to convert a first part of the sunlight
incident on the surface to electrical power and to reflect a second part
of the sunlight incident on the same surface. The at least one reflector
may be configured to direct the second part of the sunlight incident
thereon to a solar receiver. The solar power system may further include a
power management system configured to receive electrical power derived
from the first part from the reflector.

[0015]In another aspect, the solar receiver may include a photovoltaic
converter. In yet another aspect, the reflector may include co-planar
reflecting and photovoltaic conversion layers.

[0016]In another aspect, the at least one reflector may include multiple
reflectors and the solar receiver may include a solar thermal boiler. In
another aspect, the reflector may be a heliostat. In a further aspect,
the solar power system may include multiple heliostats. The reflector may
receive sunlight from the multiple heliostats.

[0017]In yet another aspect, the reflector may be configured such that the
first part has first wavelength distribution and the second part has a
second wavelength distribution.

[0018]In still another aspect, the reflector may be configured such that
the first part has first wavelength distribution and the second part has
a second wavelength distribution. The first wavelength distribution may
be a band between portions of the second wavelength distribution. In yet
another aspect, the second wavelength distribution may include high and
low pass ranges flanking the first wavelength distribution.

[0019]In another aspect, the surface may include a dichroic mirror. The
reflector may be configured such that the first part has first wavelength
distribution and the second part has a second wavelength distribution
responsive to the reflectance/transmittance properties of the dichroic
mirror.

[0020]In another aspect, the reflector may have alternating reflecting and
photovoltaic conversion layers. The alternating layers may be configured
to permit the partial transmission of light through at least one of the
layers to another of the layers.

[0021]In another aspect, the reflector may be at least part of a heliostat
whose surface is dished to concentrate light reflected therefrom. In
another aspect, the reflector may be at least part of a parabolic trough.
In yet another aspect, the reflector may be at least part of a
cylindrical trough. In still another aspect, the reflector may be at
least part of a parabolic or cylindrical trough. In another aspect, the
reflector may be at least part of a linear Fresnel reflector.

[0022]In a further aspect, the solar power system may include a controller
configured to selectively aim the reflector such that the reflector is
normal to incident sunlight. The controller may selectively aim the
reflector such that the reflector is normal to incident sunlight at times
when the angle formed by the receiver, the reflector and the sun, with
the reflector at the angle vertex, is greater than a predefined angle. In
another aspect, the controller may selectively aim the reflector such
that it is normal to incident sunlight at times when the production of
the system peaks due to peak insolation levels and to reflect sunlight to
the solar receiver at other times. In still another aspect, the
controller may be configured to selectively aim the reflector such that
it is normal to incident sunlight at times when the angle formed by the
receiver, the reflector and the sun, with the reflector at the angle
vertex, is greater than a predefined angle. The controller may be further
configured to selectively aim the reflector such that it is normal to
incident sunlight at times when the total amount of flux available to the
system is above a certain level. In still another aspect, the controller
may be configured to selectively aim the reflector such that it is normal
to incident sunlight at times when the total amount of flux available to
the receiver is above a certain level.

[0023]In another aspect, the reflector may be configured such that the
first part has first wavelength distribution and the second part has a
second wavelength distribution. The surface of the reflector may have a
photovoltaic converter that converts light of the first wavelength
distribution to electricity with a higher probability than wavelengths of
the second wavelength distribution.

[0024]In another aspect, the reflector surface may include a reflecting
surface and a photovoltaic layer on a transparent substrate overlying the
reflecting surface. In another aspect, the reflector surface may include
a reflecting surface and a photovoltaic dye layer on a transparent
substrate overlying the reflecting surface. In still another aspect, the
reflector surface may include a photovoltaic layer on a reflective
substrate. In another aspect, the reflector surface may include a
photovoltaic transparent thin film on a reflective substrate. In yet
another aspect, the reflector surface may include a photovoltaic
amorphous silicon thin film on a reflective substrate.

[0025]In a further aspect, the solar power system may include a controller
configured to selectively aim the reflector such that it reflects
sunlight onto the receiver or faces the reflector surface toward the sun
responsively to at least one of parameters indicating instant electricity
production of the system, revenue or profit generation of the system,
instant energy conversion efficiency of the system, alternatively
electricity production of the system, and revenue generation or profit
generation of the system cumulated over a time period

[0026]In a further aspect, the solar power system may include additional
reflectors configured to reflect all the sunlight received thereby. In a
further aspect, the solar power system may include additional reflectors
configured to reflect all the sunlight received thereby to the receiver.

[0027]In a further aspect, the solar power system may include
non-photovoltaic heliostats. The reflector may be one of multiple
photovoltaic heliostats. The non-photovoltaic heliostats may be
positioned relative to the receiver so as to form less acute angles with
the sun and the receiver during a greater fraction of a production period
of the year than the photovoltaic heliostats. The positions of the
photovoltaic heliostats and non-photovoltaic heliostats may be chosen
such that the angle of incidence on the photovoltaic heliostats tends to
be lower while reflecting sunlight to the receiver than the
non-photovoltaic heliostats.

[0028]In a further aspect, the solar power system may include
non-photovoltaic heliostats. The reflector may be one of multiple
photovoltaic heliostats. Each photovoltaic heliostat may have an optical
acceptance angle. The non-photovoltaic heliostats may be located in
positions such that, when reflecting sunlight toward the receiver, they
form angles with the sun and the receiver during a substantial fraction
of a production period that is lower than the optical acceptance angle.
The positions of the photovoltaic heliostats and non-photovoltaic
heliostats may be chosen to provide a highest possible utilization factor
of the photovoltaic conversion capability of the photovoltaic heliostats.

[0029]An embodiment of the present invention may include a mechanism for
accepting energy from a source and redirecting the energy therefrom in
response to certain conditions. The redirecting the energy may serve to
compensate a central energy production system and to take maximum
advantage of the energy that is available.

[0030]In an aspect, the source is the sun and the energy from the source
is solar radiation. In another aspect, the mechanism for accepting and
redirecting energy is a reflector. In yet another aspect, the mechanism
is a panel that provides integrated reflection and energy conversion.

[0031]In another aspect, one or more energy redirectors may also have the
ability to convert accepted energy directly into some other form of
energy themselves. In an alternative aspect, the one or more energy
redirectors may also have the ability to direct the energy selectively to
alternative converters.

[0032]In still another aspect, the redirectors may split the energy into
different bands and direct the energy toward a converter that can make
the most efficient use of it. In an aspect, the energy may be split such
that one waveband is directed to a photovoltaic converter and another
waveband is directed to a thermal converter.

[0033]In yet another aspect, the redirector may be a mirror. In another
aspect, the redirector may be a prism. In another aspect, the redirector
may be any other suitable energy redirector.

[0034]In another aspect, a controller may be provided which functions to
redirecting the energy under predefined circumstances.

[0035]In another aspect, the mechanism for accepting energy and
redirecting the energy may serve to recapture otherwise wasted excess
total system flux.

[0036]In another aspect, the mechanism for accepting energy and
redirecting the energy may include a central mirror with variable
reflectivity. The central mirror may rapidly modulate the total flux to
guard against emergency conditions.

[0037]In yet another aspect, the mechanism for accepting energy and
redirecting the energy may capture energy when there is low direct
insolation but high total diffuse insolation. The energy may be captured
by distributed photovoltaic converting heliostats.

[0038]In still another aspect, the mechanism for accepting energy and
redirecting the energy may moderate the flux during an unsteady operating
period of the receiver but capture the energy that would otherwise be
lost. In an aspect, the unsteady operating period of the receiver may be
a startup period of a boiler.

[0039]In still another aspect, the mechanism for accepting energy and
redirecting the energy may make use of solar energy at wavelengths that
are illsuited to conversion by economical photovoltaic devices. In an
aspect, the solar energy at wavelengths ill-suited to conversion by
economical photovoltaic devices may be directed to a different type of
converter such as thermal receiver of a concentrating system.

[0040]Objects, advantages, and novel features of the present invention
will become apparent from the following detailed description of the
invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIGS. 1 and 2 are diagrammatic elevation views of a plurality of
heliostats and a central power tower in accordance with different
embodiments of the invention.

[0042]FIG. 3a is a diagrammatic elevation view of a heliostat and power
tower illustrating the incidence and reflection angles of solar radiation
with respect to focusing on the tower.

[0043]FIG. 3b is a diagrammatic elevation view similar to FIG. 3a, showing
the same heliostat pointing toward the sun in another operating mode.

[0044]FIGS. 4 and 5 are diagrammatic cross-sections of heliostat mirrors
configured for both reflection and photovoltaic conversion in accordance
with preferred embodiments of the invention.

[0045]FIGS. 6a and 6b illustrate a heliostat with features relating to a
photovoltaic conversion capability.

[0046]FIG. 6c illustrates a field of heliostats with subgroups with
assigned members to generate electrical power for the subgroup.

[0050]FIG. 11 is a diagrammatic elevation view of a plurality of
heliostats, an intermediate reflector with an energy converter, and a
central receiver.

[0051]FIG. 12 is a diagrammatic elevation view of a plurality of
heliostats, a pair of intermediate reflectors, and a pair of receivers.

DETAILED DESCRIPTION OF THE INVENTION

[0052]A concentrating solar power (CSP) system uses solar radiation that
is concentrated by reflecting or otherwise focusing incident insolation
from a larger area, usually comprising mirrors but optionally including a
lens, onto a smaller area. The smaller area preferably comprises a
receiver where the radiation is converted to some useful form, such as
heat which is optionally used for the production of electricity or by
directly converting the sunlight to electricity such as by photovoltaic
conversion. Most CSP systems include tracking or calculating the apparent
position of the sun at various intervals in order to achieve a desired
angle of reflection that can utilize the direct normal component of the
solar radiation.

[0053]A CSP may include at least one solar power tower and at least one
set of heliostats equipped with mirrors for reflecting solar radiation
onto a receiver positioned on the tower. Alternatively, a CSP system may
include a plurality of parabolic trough mirror assemblies which reflect
solar radiation onto a receiver, such as a fluid-conveying pipe, situated
above, axially aligned with, and in the focus of, the parabolic trough.
In another alternative configuration, a CSP system may include a
hyperbolic or parabolic dish comprising multiple mirror facets which
reflect solar radiation onto a receiver at the focus. In yet another
alternative configuration, a plurality of substantially flat mirrors are
assembled to form a kind of ad hoc single-axis Fresnel lens that reflects
sunlight onto receivers, such as fluid-conveying pipes or concentrated
photovoltaic modules, which are positioned above, axially aligned with,
and in the focus of, the ad hoc Fresnel lens.

[0054]In some embodiments, the CSP system includes a solar power tower
system wherein a central receiver is located at the top of a receiver
tower or at some other location, for example if an intermediate reflector
is used to bounce light received at the top of a tower down to a receiver
located at ground level. Referring now to the figures and in particular
to FIG. 1, a solar power system 44 is provided in which heliostats 38
include mirrors 8 that reflect incident solar radiation 28 onto a
receiver 1 in which a working fluid (not shown) is heated for later use
in an electric power generating plant 45, and/or in which electricity is
generated by photovoltaic conversion of reflected light. The
heliostat-mounted mirrors 8 are capable of tracking the apparent movement
of the sun 25 across the sky each day in order to maintain the reflective
focus in the direction of the receiver 1 as the angle of the incident
radiation 28 changes. The receiver 1 is located atop a tower 43, or in an
alternative embodiment, shown in FIG. 2, is located on the ground, and
the heliostat-mounted mirrors 8 reflect solar radiation onto one or more
suspended mirrors 9 which further reflect the radiation onto the receiver
1.

[0055]According to a preferred embodiment, at least some of the heliostats
are capable of reflecting at least some of the incident solar radiation
onto a receiver and also of converting at least some of the incident
solar radiation directly into electricity through photovoltaic
conversion. In this specification photovoltaic conversion has its usual
meaning but may also be taken to include the generation of electricity
from solar energy by electrochemical means in a dye-sensitized solar cell
or similar arrangement. In some embodiments a heliostat can alternately
perform one function (reflection) or the other (photovoltaic conversion)
at different times, and in other embodiments a heliostat can perform both
functions at the same time. This is accomplished with a heliostat that
includes alternating reflecting and photovoltaic conversion layers, as
described in the embodiments, which allow the partial transmission of
light through at least one layer to another.

[0056]Non-concentrating photovoltaic systems are known in the art and can
be configured in various ways, including flat-plate horizontal,
flat-plate tilted, single-axis tracking, and double-axis tracking.
Double-axis tracking is known to provide the highest energy conversion
efficiency of any of these because the photovoltaic cells are made to
track, with two axes of movement, the apparent movement of the sun across
the sky so as to point directly toward the sun at all times. This exposes
the photovoltaic cells to both diffuse solar radiation and direct normal
insolation, that is, direct insolation while the photovoltaic collector
is aligned in a plane normal to the vector of the incident radiation.

[0057]However, conversion of solar radiation to electrical energy by
direct in-situ photovoltaic conversion to electrical energy, even with
double-axis tracking, is generally less efficient than a solar power
tower system that involves reflection of incident solar radiation onto a
receiver capable of converting the reflected radiation into heat in a
fluid which is then transported to an electricity generating means such
as a turbine that can convert thermal energy in the heated fluid to
electricity. Similarly, conversion of solar radiation to electrical
energy by direct in-situ photovoltaic conversion to electrical energy is
generally less efficient than a solar power tower system that includes
concentrated photovoltaic modules, which involves reflection onto a
receiver capable of converting the reflected and concentrated radiation
directly into electricity by means of high-efficiency concentrated (i.e.,
multi-junction or multi-bandgap) photovoltaic cells.

[0058]Nonetheless, there are times when the instant efficiency of
reflecting solar radiation from certain heliostats in a solar power tower
system to a receiver can be less than the instant efficiency of
converting that solar radiation to electrical energy by direct in-situ
photovoltaic conversion. For example, a heliostat located to the east of
a receiver in the northern hemisphere will reflect incident solar
radiation on the receiver with relatively large cosine losses during the
early morning hours. Cosine losses in a tower system occur because the
effective reflection area of a heliostat is reduced by the cosine of
one-half of the angle between incident radiation and reflected radiation.
During those morning hours, the angle between incident radiation and
reflected radiation is substantially more than 90 degrees and therefore
cosine losses significantly impact on the energy conversion efficiency of
energy impinging on such a heliostat. In a first mode of operation, such
a heliostat equipped with photovoltaic conversion means in accordance
with any embodiment of the present invention, could be directed by a
control system or system operator to turn toward the apparent position of
the sun in the sky instead of its normal tracking position of halfway
between the sun and the receiver, and convert both diffuse and direct
insolation to electricity with a higher energy conversion efficiency.

[0059]Referring again to the figures and in particular to FIGS. 3a and 3b,
FIG. 3a illustrates the situation wherein the sun 25 is relatively low in
the morning sky, and angle θ defines the angle between incident
radiation 28 and reflected radiation 29, which is reflected, by mirror 8
mounted on heliostat 38, onto receiver 1 on tower 43. Angle θ is
substantially larger than 90 degrees and the attendant cosine losses mean
that the efficiency of solar-to-electric energy conversion with respect
to this heliostat is accordingly lowered. FIG. 3b shows that the
heliostat-mounted mirror 8 can be turned, alternatively, to face the sun
25 directly in order to receive direct normal insolation, as well as
diffuse insolation, for direct, in-situ photovoltaic conversion if the
heliostat-mounted mirror 8 is configured in accordance with one of the
embodiments described herein.

[0060]There are also times in the normal operation of a solar power tower
system when a heliostat is directed to turn away from the sun and not
reflect light onto a receiver because the incremental energy reflected by
the heliostat would cause the receiver to exceed its rated output. This
occurs because solar power systems are not sized in accordance with the
peak solar conditions of, for example, solar noon on a summer day, but
rather in accordance with an economic optimization that involves a
tradeoff between excess capacity during times of peak solar radiation and
inadequate collected solar radiation at other times. The distribution of
solar radiation over the hours of the year tends to have a normal
distribution peaking in summer, and therefore a solar power system sized
to provide a given rated energy output at less than peak solar radiation
will always have excess heliostat capacity during some peak hours.
Instead of simply turning these heliostats toward the ground, as is
common practice in solar-thermal only or concentrated photovoltaic
plants, such heliostats could be directed instead to turn directly toward
the sun (rather than at a angle that would reflect incident radiation
onto the receiver) and convert both diffuse and direct normal insolation
to electricity through photovoltaic conversion.

[0061]In another mode of operation, a heliostat in accordance with the
present invention can be directed to track the sun so as to reflect
incident solar radiation onto a receiver. At the same time, the
photovoltaic means installed on the heliostat can convert some of the
incident solar radiation directly to electricity, including diffuse
radiation and direct radiation, although the amount of direct radiation
converted will be reduced in this mode in accordance with the cosine of
half of the angle between the incident radiation and the reflected
radiation (angle θ in FIG. 3a).

[0062]Another mode of operation may be provided under conditions where
clouds scatter the light and prevent effective operation of a solar
thermal concentrating system employing heliostats. Under such conditions,
photovoltaic conversion heliostats can be positioned at an angle
optimized for photovoltaic conversion, for example, of diffuse light. For
example, the heliostats may be positioned to face the sun or positioned
horizontally or any other position depending on which affords the highest
electrical generating rate.

[0063]Another mode of operation may be provided to compensate for
non-uniform illumination of the photovoltaic layer, or layers of multiple
heliostats, which could cause electrical problems depending on how the
portions of the layer are electrically interconnected or the heliostats
are electrically interconnected. The mode may change heliostats with
photovoltaic conversion to full reflection or otherwise electrically
compensate during such times. Such non-uniform illumination could occur
at certain times of the day or year when some heliostats cast shadows on
others, for example.

[0064]In order for a heliostat reflector to be capable of both reflection
and photovoltaic conversion, the incident light may be passed through a
reflecting layer to reach a photovoltaic conversion layer or
alternatively, to pass through a photovoltaic conversion layer to reach a
reflecting layer to pass through the photovoltaic conversion layer again.

[0065]In an embodiment, a heliostat mirror includes at least a
photovoltaic conversion layer and a partially reflecting layer, where the
partially reflecting layer is positioned between the sun and the
photovoltaic conversion layer. Referring to FIG. 4, a heliostat mirror 8
includes an optional front glass layer 110, a partially reflecting layer
112, a photovoltaic conversion layer 111 and a backing layer 113. The
optional front glass layer 110 is preferably low-iron float glass with a
thickness between 2 and 6 mm but can be made of any other acceptable
material, such as a transparent polymer or ceramic material. The backing
layer 113 is configured for protection of the photovoltaic conversion
layer 111 and optionally for facilitating attachment to mounting or
support elements for installation on a heliostat, and may contain metal
or polymeric materials, or both in combination or in layers. The
photovoltaic conversion layer 111 is optimally connected via a conductor
114 (for example, an ohmic conductor) to an external load such as a
transformer or a motor.

[0066]In a preferred embodiment, the partially reflecting layer 112 is
spectrally selective. For example, a dichroic filter may reflect light at
certain wavelengths while transmitting light at other wavelengths. As is
known in the art, by suitable selection of the thickness and number of
layers, the wavelength range of the passband (or waveband) of such a
filter can be determined. In a preferred embodiment, the passband, the
portion of the spectrum that is transmitted by (or passed through) the
spectrally-selective partially reflecting layer, is chosen to
substantially coincide with or overlap the portion of the spectrum that
is most effectively converted by the photovoltaic conversion layer 111,
for example, the band associated with the bandgap of the semiconductor
component of the photovoltaic conversion layer. By limiting the arrival
of light that is outside the bandgap (mainly the portion of the spectrum
where photon energy is less than the bandgap) the amount of heat buildup
in the photovoltaic conversion layer can be reduced, while at the same
time making the other wavelengths available for reflection when the
mirror is deployed in a reflecting mode.

[0067]The partially reflecting layer 112 may be provided, in one or
multiple applications, by thin film deposition on the photovoltaic
conversion layer. In a particularly preferred embodiment, the
photovoltaic conversion layer is provided by thin film deposition on the
backing layer 113, and the partially reflecting layer is also formed on
over photovoltaic conversion layer 111. The photovoltaic conversion layer
111 and reflecting layer 112 can be formed by vapor deposition, printing
or any suitable process.

[0068]In another embodiment, a heliostat mirror includes at least a
photovoltaic conversion layer and a reflecting layer, where the
photovoltaic conversion layer is positioned between the sun and the
reflecting layer. Referring to FIG. 5, a heliostat mirror 8 includes an
optional front glass layer 110, a photovoltaic conversion layer 111 which
partially transmits light, a reflecting layer 112 that is preferably a
perfect reflector, and a backing layer 113. The photovoltaic conversion
layer 112 is optimally connected via a conductor 114 to an external load
such as a transformer. The photovoltaic conversion layer 111 is at least
partially transparent and can be formed as a thin film on a separate
substrate or formed on the reflecting layer 112. An example of a suitable
material for a partially transparent photovoltaic conversion layer is
Power Glass, developed by Xsunx, Inc., of Aliso Viejo, Calif.
Alternatively, the photovoltaic conversion layer can employ dyes or ink
applied by a suitable process on the reflecting layer. Any method of
forming such a layer is suitable and may be formed on materials such as
glass, metal or plastic, any of which could be used in a reflecting
layer.

[0069]The photovoltaic conversion layer 111 may be configured to convert
at least a portion of incident light to electricity before it reaches the
reflecting layer 112 and to convert at least a portion of reflected light
to electricity after it has been reflected by the reflecting layer 112.
The portion of the incident light reflected and subsequently
photovoltaically converted may be in the same portion of the spectrum
that was photovoltaically converted before being reflected, or it may
overlap that portion of the spectrum, or it may be in a different portion
of the spectrum. For example, the reflecting layer can include a
fluorescing filter that alters the wavelength of at least a portion of
the light reaching the reflecting layer. In another example, a phosphor
or wavelength converting layer may be provided adjacent to the
photovoltaic layer to thereby convert incident solar radiation into
wavelengths at which the photovoltaic layer is responsive. In yet another
example, phosphor or wavelength converting particles can be incorporated
within the photovoltaic or reflecting layer. In other embodiments, the
reflecting layer 112 of FIG. 4 may include a liquid crystal layer
configured to transmit or reflect light depending on its state.

[0070]In all of the embodiments, where possible, the discussions relating
to heliostats FIG. 1, reference numeral 8, and central reflector
embodiments FIG. 2, reference numeral 9, are interchangeable. That is,
all of the features described with regard to a reflector/photovoltaic
converter can be provided in the heliostat 8, the central or intermediate
reflector 9, or both.

[0071]One mechanism for controlling the state of the reflector is by using
a local algorithm implemented through a microcontroller. For example, if
the ratio of the intensity of diffuse light (light coming from many
directions) to the intensity of direct light (light from a single
direction) exceeds a threshold, the reflector may switch from reflecting
to photovoltaic mode. The local controller may also employ time of day
information, total insolation, and other information to control its
state.

[0072]Equipping a heliostat with a dual-function mirror in accordance with
any of the preceding embodiments may cost more than a simple reflecting
mirror as well as reduce the total reflectivity of any such mirror to
some degree, and therefore it may be preferable to have a select subset
of heliostats in a solar power tower system to be so equipped. Preferably
the subset is chosen according to an optimization scheme that takes
account of the acceptance angle limitations of the photovoltaic portion
of the reflector. For example, some heliostats will provide more total
photovoltaic conversion over a period of time because the incident light
is within the range of acceptance angles of the photovoltaic converting
layer for a greater fraction of that period of time whilst the heliostat
is in a mode in which it is tracking the sun for concentrating it on a
central receiver. For example, if the system is configured such that the
main contribution of photovoltaically-converted energy is from heliostats
to the east of a receiver in the early morning and from heliostats to the
west of a receiver in the late afternoon, then it may be economically
feasible to equip those heliostats with the dual-function mirrors but not
others. In an alternative embodiment, it is possible to preselect the
heliostats which would not be needed for reflection of solar radiation
onto a receiver during peak summer hours because of excess capacity at
peak, and then to equip those heliostats with the dual-function (both
reflection and photovoltaic conversion) mirrors. In another alternative
embodiment, all of the heliostats in a solar power tower system are so
equipped.

[0073]Heliostats may be configured to allow rapid removal and replacement
on their supports. In a variation of this concept, the heliostats are
configured to allow the components associated with the photovoltaics to
be removed and transported with the heliostat in a convenient manner with
low risk of misconnection and damage. For example, a power management
module, energy storage, controller, etc. could be combined in a single
housing. In a preferred method, the heliostats are moved around during
the course of the year as the solar declination angle changes to allow
heliostats with photoconversion to be positioned where the sun angle is
within the acceptance angle for a greater fraction of the day. For
example, this could be done according to a regular schedule on the order
of weeks or months apart depending on the rate of change of declination
angle.

[0074]Referring to FIGS. 6a and 6b, in embodiments, electricity generated
by a photovoltaic layer 204 in a heliostat reflector 200 in accordance
with any of the embodiments herein is used for powering at least one
positioning drive 206 of the heliostat reflector 200. Power may be
supplied to a power management unit 202 which may have a controller
configured to control the heliostat drive 206 autonomously or
semi-autonomously. Energy may be stored locally, semi-locally (See FIG.
6c and attending discussion) or centrally. Energy storage 208 may be
provided by a battery, ultracapacitor, flywheel, thermal storage means,
or any other suitable device. In a further embodiment, the electricity is
used for providing electricity to a system power management system 212
which may provide power to other loads such as a recirculation pump in a
receiver boiler 214. In a variation, the electricity is transmitted to a
transformer or rectifier and sold. Referring to FIG. 6c, power from a
photovoltaic-generating heliostat 230 may be provided to other heliostats
231 in each subset 232 of a field 228. Each subset may have a respective
energy store 233.

[0075]FIG. 7 illustrates a reflector device 400 with a back element 422
and an overlying elements 420 that can be selectively deployed; placed
over the back element 422 as indicated at 424 (solid lines); or stored as
indicated at 426 (broken lines). The overlying elements 420 may be opaque
or transparent photovoltaic elements used in combination with the back
element 422 having a reflective surface. The back element 422 may be a
reflective (mirror) element, a photovoltaic element, or a reflective
element with a photovoltaic layer element overlying it. The overlying
elements 420 may be waveband filters, such as dichroic filters, with
photovoltaic layer on the back element 422 in another embodiment.
Alternatively, the overlying element may be transparent or opaque
photovoltaic elements with the back element 422 being a reflector. The
above configuration may be used in heliostats FIG. 1, reference numeral
8, central reflectors FIG. 2, reference numeral 9, or both.

[0076]In an alternative configuration, a variable reflecting layer may
overlie the backing member. The variable reflecting layer may be provided
between the photovoltaic conversion layer and the sun, so as to regulate
the amount of light reaching the photovoltaic conversion layer versus the
amount of light reflected. For example, at times when maximum reflection
is desired, the variable reflecting layer can be adjusted so as to
reflect most of the light incident thereon. Conversely, at times when
maximum photovoltaic conversion is desired, the variable reflecting layer
may be adjusted so as to transmit most of the light incident thereon to
the photovoltaic conversion layer.

[0077]Such a variable reflecting layer could be embodied in an LCD window.
In another variation, a variable reflecting layer may be provided by one
or more antireflection (quarter wavelength) films overlying a transparent
substrate, where the film has a selectable index of refraction. In still
another variation, a variable reflecting layer may be achieved by a
plurality of dielectric films of a specific thickness designed to reflect
a specific waveband, wherein the reflectance waveband may be shifted
using electrical means. Such a feature may be achieved by employing
materials that have a thermo-optic or electro-optic effect. Thus, an
electric field or current applied to the film may effect a change in the
index of refraction sufficient to alter the reflectivity of the film at
certain wavelengths. In yet another variation, an electrochromic
reflecting layer could be employed for the variable reflector. As
discussed, for example in U.S. Pat. No. 7,042,615 to Richardson, which is
hereby incorporated by reference in its entirety, a variable reflectance
layer can be realized by an electrochromic device, which transitions from
a highly reflecting metallic state to a mainly transparent state by
application of an appropriate electric field. Other examples of variable
reflectors suitable for use in the present system can be found in U.S.
Pat. No. 5,905,590 to Van Der Sluis et al. and U.S. Pat. No. 6,310,725 to
Duine et al., both of which are hereby incorporated by reference in their
entirety. The above configurations may be used in heliostats FIG. 1,
reference numeral 8, central reflectors FIG. 2, reference numeral 9, or
both.

[0078]FIG. 8 illustrates a trough or dish reflector 404 with a portion 406
having a photovoltaic portion. The portion 406 can have an overlying
selective waveband filter or a variable reflective surface. Preferably,
the angular size 403 of the portion 406 is chosen based on the acceptance
angle of the photovoltaic portion such that when the reflector 404 is
aimed at the sun, to focus light on the receiver 402, the sun's rays are
within the acceptance angle of the photovoltaic portion 406. The
reflector can be a Fresnel reflector, faceted, multi-element reflector
with equivalent features and function in an alternative configuration.
FIG. 9 shows a reflector trough or dish 401 in which the entirety of the
reflector 412 contains a photovoltaic layer which may be accompanied by
an overlying selective waveband filter or a variable reflective surface.

[0079]In other embodiments, a CSP system that is not a solar power tower
system, as described above, includes mirror facets which are capable of
reflecting at least some of the incident solar radiation onto a receiver
and also of converting at least some of the incident solar radiation
directly into electricity through photovoltaic conversion. The mirror
facets include alternating reflecting and photovoltaic conversion layers,
as described in the various embodiments above, which allow the partial
transmission of light through at least one layer to another. The mirror
facets can be configured in accordance with any of the embodiments
described for heliostat mirrors, e.g., with a photovoltaic conversion
layer between the reflecting layer and the sun, or vice versa, such as
those illustrated in FIGS. 4 and 5.

[0080]In an example, a CSP system includes a plurality of parabolic trough
assemblies superficially similar to the LS-3 collectors installed at the
SEGS IX trough plant at Harper Lake, Calif., the design details of which
are available on the website of the National Renewable Energy Laboratory.
The LS-3 collector design employs 224 mirror panels in each collector
assembly, and these mirror panels aggregately form an approximation of a
one-curve solid parabola so that most of the solar radiation incident on
the mirror panels reflects onto a heat collection pipe, acting as a
receiver for reflected solar energy, which runs axially along the length
of the assembly at the approximate focus of the parabola. According to
the example, each of the mirror panels includes a reflecting mirror on
the face of which has been printed an array of CIGS nanoparticles
comprising a photovoltaic layer. The direct-current electrical output of
the photovoltaic layers of the mirror panels in the assembly is used for
powering the assembly's drive motor. In another example, only some of the
mirror panels are equipped with a photovoltaic conversion layer. In yet
another example, the direct current electrical output of the collector is
used elsewhere in the CSP plant or sold to an electric utility after
optionally being converted to alternating current electricity.

[0081]In another example, a CSP system includes a plurality of
substantially flat mirrors assembled to form a kind of ad hoc single-axis
Fresnel lens that reflects sunlight onto receivers which include
concentrated photovoltaic modules, where the receivers are positioned
above, and are axially aligned with and in the focus of, the ad hoc
Fresnel lens. A suitable assembly is the Compact Linear Fresnel Reflector
developed by Solar Heat and Power Pty Ltd. of Australia. According to the
example, the receiver includes modules incorporating multi-junction or
multi-bandgap photovoltaic cells, a suitable example of which is the
EMCORE T1000 Triple-Junction High-Efficiency Solar Cell available from
EMCORE Photovoltaics of Albuquerque, N. Mex. The flat mirrors are
constructed in accordance with the embodiment illustrated in FIG. 4,
wherein a photovoltaic conversion layer comprising a cadmium-telluride
polycrystalline thin-film deposition is interposed between a backing
layer and a spectrally selective mirror. The flat mirrors can be operated
in various operating modes as described earlier with respect to
heliostats in a solar power tower system, including a first operating
mode of tracking the sun for reflection on the receiver, where radiation
utilized for photovoltaic conversion is reduced by the cosine of the half
the angle between the sun and the reflector; and a second operating mode
of tracking the sun so as to face the sun, i.e., to utilize both diffuse
and direct normal radiation. In the second operating mode, radiation
impinging on the flat mirrors will be reduced by the cosine of the sun
angle with respect to the horizontal disposition of the flat mirrors, but
this cosine loss is nonetheless smaller than that which is typical of the
first operating mode.

[0082]Referring to FIG. 10, there are various combinations of the features
of the systems described above that will provide all or some of the
benefits described and others. FIG. 10 shows some of these combinations
in schematic form. Each of nine configurations is illustrated by a
combination of icons (e.g., one such icon is indicated at 500). Each icon
represents a component or component class, namely, a primary reflector or
reflectors 520, for example, a field of heliostats, a secondary element
521 such as a reflector 9 in FIG. 2, and a final receiver 522. Each icon
represents one or more possible combinations of subelements which are
indicated in the legend 540. Specifically, "F" indicates a waveband
filter which may overly a photovoltaic converter, for example. "V"
indicates a variable reflector providing selectable degree of reflection
and transmission. "P" indicates a photovoltaic converter as well as a
photovoltaic converter with a reflective backing. In the latter
variation, the photovoltaic converter would be at least partly
transparent. Thus "P" represents two possible configurations. "R"
represents a reflector. "R" in a dashed line box represents a reflector
or no reflector as in a direct aiming system that does not employ an
intermediate reflector (e.g., FIG. 1 or FIG. 2 configuration) "X"
represents light passing through and being lost such as when a variable
intermediate reflector is used to dump some of the energy to free space.
"Z" represents a high density photovoltaic converter or thermal converter
such as a boiler (See FIG. 11 and attending discussion below). "T"
represents thermal conversion.

[0083]Configuration 502 has one or more primary reflectors with one or
more of them having a photovoltaic conversion layer. The primary
reflectors may be combined with a secondary reflector or the secondary
reflector may be left out. A thermal receiver receives light reflected by
the primary reflectors and the secondary element if present.

[0084]Configuration 504 has one or more primary reflectors with one or
more of them having a photovoltaic conversion layer with an overlying
filter, for example a waveband filter. The primary reflectors may be
combined with a secondary reflector or the secondary reflector may be
left out. A thermal receiver receives light reflected by the primary
reflectors and the secondary element if present.

[0085]Configuration 506 has one or more primary reflectors with one or
more of them having a photovoltaic conversion layer with an overlying
variable reflector. The primary reflectors may be combined with a
secondary reflector or the secondary reflector may be left out. A thermal
receiver receives light reflected by the primary reflectors and the
secondary element if present.

[0086]Configuration 508 has one or more primary reflectors. The primary
reflectors may be combined with a secondary reflector or the secondary
reflector may be left out. A thermal receiver with a receiving surface
having an overlying photovoltaic conversion layer receives light
reflected by the primary reflectors and the secondary element if present.
The receiver converts thermal energy through the photovoltaic material
overlying the receiver.

[0087]Configuration 510 has one or more primary reflectors. The primary
reflectors may be combined with a secondary reflector having a
photovoltaic conversion layer. A thermal receiver receives light
reflected by the primary reflectors and the secondary element if present.

[0088]Configuration 512 has one or more primary reflectors. The primary
reflectors may be combined with a secondary reflector having a
photovoltaic conversion layer with an overlying filter, for example a
waveband filter. A thermal receiver receives light reflected by the
primary reflectors and the secondary element if present.

[0089]Configuration 514 has one or more primary reflectors. The primary
reflectors may be combined with a secondary reflector having a
photovoltaic conversion layer with an overlying variable reflector. A
thermal receiver receives light reflected by the primary reflectors and
the secondary element if present.

[0090]Configuration 516 has one or more primary reflectors. The primary
reflectors may be combined with a secondary reflector having a thermal
conversion element with an overlying variable reflector. The thermal
conversion may be used to make the secondary element into an additional
receiver to pick up and convert excess flux to thermal energy. For
example, when the system is experiencing a peak insolation period,
instead of dumping light by diverting heliostats, the variable reflector
can be adjusted to allow more light to pass through to the thermal
conversion element, which may be, for example, a boiler. A thermal
receiver receives light reflected by the primary reflectors and the
secondary element if present. Excess flux conditions may occur at peak
operating periods or during startup when the system could have a
predefined limit in the rate of increase of heat gain by a thermal
receiver.

[0091]Configuration 518 has a variable intermediate reflector in which
energy that is not reflected is transmitted and lost to the sky or an
absorber to dissipate it. This would be used for purposes of dumping
excess energy.

[0092]FIG. 11 illustrates a system with an intermediate reflector 609 with
an energy converter 602 provided behind the intermediate reflector 609.
This configuration is an example according to the configurations 510,
512, 514, 516, and 518 illustrated above. Energy converter 602 may be a
thermal or photovoltaic converter or any other device that can convert
solar energy to a usable form. 602 may, for example, be a boiler and the
reflector 609 may be a variable reflector. Instead of diverting
heliostats 8 when excess flux has to be dumped, the variable reflector
may permit more light to impinge upon such boiler 602 and the thermal
energy used for some useful purpose, such as preheating. Where 602 is a
photovoltaic or other energy converter, under the same conditions, the
"dumped" flux can be converted to usable form by adjusting the
reflectivity/transmissivity characteristics of the reflector 609. In a
further variation, reflector 609 is a variable reflector with nothing
behind it to absorb the incident light such that the "dumped" flux is
lost to the sky. Note that the above functions can be implemented by a
beam splitter with a variable reflectance such that the energy converter
602 can be located on the ground.

[0093]Referring to FIG. 12, in another embodiment, two intermediate
reflectors 620 and 621 are provided and can be configured in real time to
divide the flux between two receivers 615 and 617. In an application,
excess flux is directed by one of the reflectors 621 to receiver 617. In
this example, 615 could be a thermal receiver for a solar thermal power
plant with a rated capacity. When excess flux is received, the excess
flux can be diverted by tiling the reflector 621 such that part of the
energy falls on the receiver 617 instead of 615.

[0094]Certain features of this invention may sometimes be used to
advantage without a corresponding use of the other features. While a
specific embodiment of the invention has been shown and described in
detail to illustrate the application of the principles of the invention,
it will be understood that the invention may be embodied otherwise
without departing from such principles.